Misregistration correction for bidirectional printing in consideration of inclination of nozzle array

ABSTRACT

A correction value, which is used to correct misalignment of recording positions in a main scanning direction, is determined using a representative nozzle sub-array as a reference. The representative nozzle sub-array is within a predetermined range around the center of a nozzle array provided on a print head. The correction value is set, based on a positional misalignment test pattern printed with the representative nozzle sub-array. The misalignment of recording positions in the main scanning direction in the course of bidirectional printing is corrected with the correction value.

TECHNICAL FIELD

The present invention relates to a technique that carries outbidirectional, reciprocating main scan to print an image on a printingmedium. More specifically the present invention pertains to a techniquethat adjusts misalignment of dot recording positions in a main scanningdirection between a forward pass and a backward pass of the main scan.

BACKGROUND ART

Recently color printers having a print head that ejects a plurality ofdifferent color inks have been widely used as an output device ofcomputers. Some of such color printers have the function of“bidirectional printing” for enhancing the printing speed.

In bidirectional printing, misalignment of recording positions in a mainscanning direction on a forward pass and a backward pass of main scanoften arises due to a backlash of a driving mechanism in the mainscanning direction or a warp of a platen that supports a printingmedium. One of the techniques proposed to relieve such positionalmisalignment is disclosed in JPA 5-69625 filed by the applicant of thepresent invention. This prior art technique registers in advance apotential amount of positional misalignment (deviation in printing) inthe main scanning direction and corrects the dot recording positions onthe forward pass and on the backward pass, based on the registeredamount of positional misalignment.

The print head of the printer generally has a nozzle array that includesa large number of nozzles arranged in a sub-scanning direction. When theprint head is driven in the main scanning direction, mechanicalvibrations may occur on the print head to slightly shift the orientationof the nozzle array from the sub-scanning direction. Furthermore, thenozzle array may be slightly inclined to different orientations on theforward pass and on the backward pass, for example, due to the backlashof the driving mechanism in the main scanning direction. In such cases,it is difficult to accurately determine the amount of potentialpositional misalignment, since the nozzle array does not move back andforth keeping the specified posture perpendicular to the main scanningdirection as a whole.

The object of the present invention is to solve the problem of the priorart technique discussed above and accordingly to provide a techniquethat relieves positional misalignment in the main scanning direction ona forward pass and a backward pass of main scan with regard to a nozzlearray in a printing apparatus of bidirectional printing.

DISCLOSURE OF THE INVENTION

In order to attain at least part of the above and the other relatedobjects, the present invention is directed to a printing apparatus thatcomprises a print head having a nozzle array, which is arranged in asub-scanning direction and ejects ink droplets to record dots on aprinting medium, and carries out bidirectional, reciprocating main scanto complete printing on the printing medium. With this printingapparatus, a positional misalignment test pattern is printed on theprinting medium with a representative nozzle sub-array, which is part ofthe nozzle array and is within a predetermined range around a center ofthe nozzle array. Then a correction value is determined according tocorrection information that represents a favorable correction stateselected based on the positional misalignment test pattern, where thecorrection value is used to correct the misalignment of recordingpositions in the main scanning direction on the forward pass and thebackward pass of the main scan. The misalignment of recording positionsin the main scanning direction is actually corrected with the correctionvalue thus determined in the course of bidirectional printing. Here “therepresentative nozzle sub-array that is within a predetermined rangearound a center of the nozzle array” represents a group of pluralnozzles that includes a specific nozzle closest to the center positionalong the length of the nozzle array and does not include end nozzles oneither end of the nozzle array.

In the case where the angle of the nozzle array in the main scanningdirection on the forward pass is slightly different from that on thebackward pass, a correction value determined using a nozzle on one endof the nozzle array would cause a significant misalignment of recordingpositions of ink droplets with regard to the nozzles on the other end ofthe nozzle array. The arrangement of the present invention determinesthe correction value on the basis of the nozzles in the neighborhood ofthe center of the nozzle array. Accordingly, it reduces the totalmisalignment of dot recording positions, due to the variation in theangle of the nozzle array. The technique of the invention determines thecorrection value according to the correction information whichrepresents the favorable correction state selected based on thepositional misalignment test pattern printed with the representativenozzle sub-array. The correction value is determined not on the basis ofdeductive inference but on the basis of the positional misalignment testpattern actually printed on a printing medium. The correction value canthus be determined adequately to relieve the actual printingmisalignment.

The nozzle array preferably comprises: a color nozzle array including aplurality of color nozzles to eject color ink that are arranged in apredetermined sequence in the sub-scanning direction; and a black nozzlearray including a plurality of black nozzles to eject black ink that arearranged in a predetermined sequence in the sub-scanning direction. Inthis application, a memory included in the printing apparatus stores afirst correction value and a second correction value therein. The firstcorrection value is set for correcting the positional misalignment ofrecording positions in the main scanning direction on the forward passand the backward pass of the main scan with regard to a representativecolor nozzle sub-array, which is part of the color nozzle array and iswithin a predetermined range around a center of the color nozzle array.The second correction value is set for correcting the positionalmisalignment of recording positions in the main scanning direction onthe forward pass and the backward pass of the main scan with regard to arepresentative black nozzle sub-array, which is part of the black nozzlearray and is within a predetermined range around a center of the blacknozzle array. Here the color nozzle array and the black nozzle array mayhave any positional relationship.

This arrangement corrects the misalignment of recording positions in thecourse of bidirectional printing with the first correction value, whichreflects the characteristics of the color nozzle array, and with thesecond correction value, which reflects the characteristics of the blacknozzle array. Namely the arrangement of using both the first correctionvalue and the second correction value can reflect the characteristics ofthe color nozzle array and the black nozzle array on the correction ofthe misalignment of recording positions in the course of bidirectionalprinting.

The positional misalignment correction unit may correct the misalignmentof recording positions in the main scanning direction in the course ofbidirectional printing with regard to the nozzle array with a mean valueof the first correction value and the second correction value. Thisarrangement readily corrects the misalignment of recording positions inthe course of bidirectional printing by taking into account both thecolor nozzle array and the black nozzle array.

It is preferable that the positional misalignment correction unitcorrects the misalignment of recording positions with the firstcorrection value in a print mode that the nozzles of the color nozzlearray are used. In the case of color printing, this arrangement carriesout the correction with the first correction value that reflects thecharacteristics of the color nozzle array. This accordingly ensures thecorrection of recording positions suitable for color printing.

It is also preferable that the positional misalignment correction unitcorrects the misalignment of recording positions with the secondcorrection value in a print mode that the nozzles of the color nozzlearray are not used. In the case of monochromatic printing, thisarrangement carries out the correction with the second correction valuethat reflects the characteristics of the black nozzle array. Thisaccordingly ensures the correction of recording positions suitable formonochromatic printing.

The positional misalignment correction unit may correct the misalignmentof recording positions with the first correction value with regard tothe color nozzle array, and corrects the misalignment of recordingpositions with the second correction value with regard to the blacknozzle array. This arrangement ensures the optimum corrections for boththe color nozzle array and the black nozzle array in the process of oneprinting operation.

It is preferable that the color nozzle array includes yellow nozzles forejecting yellow ink, cyan nozzles for ejecting cyan ink, and magentanozzles for ejecting magenta ink, and that the representative colornozzle sub-array consists of either of cyan nozzles and magenta nozzles.

In the arrangement of determining the correction value on the basis ofthe representative nozzle sub-array that is within the predeterminedrange in the neighborhood of the center of the nozzle array, the nozzlesin the neighborhood of both the ends of the nozzle array have a greaterdegree of misalignment of dot recording positions in the main scanningdirection than the nozzles in the neighborhood of the center of thenozzle array. Among yellow, cyan, and magenta, the misalignment ofrecording positions in yellow is least conspicuous. The misalignment ofrecording positions in magenta and cyan is more conspicuous than thatin. yellow. In this preferable structure, the cyan nozzles or themagenta nozzles are arranged in the neighborhood of the center of thenozzle array and the correction value is determined with either the cyannozzles or the magenta nozzles as the representative color nozzlesub-array. This arrangement makes the total misalignment of recordingpositions of dots sufficiently inconspicuous.

The present invention is realized by a diversity of applications asgiven below:

(1) Bidirectional printing apparatus;

(2) Method of bidirectional printing;

(3) Method of correcting misalignment of recording positions in thecourse of bidirectional printing;

(4) Computer programs to attain any of the above apparatus and methods;

(5) Recording media in which computer programs to attain any of theabove apparatus and methods is recorded; and

(6) Data signals that include computer programs to attain any of theabove apparatus and methods and are embodied in carrier waves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the structure of a printing systemincluding an ink jet printer 20 in a first embodiment;

FIG. 2 is a block diagram showing the structure of a control circuit 40included in the printer 20;

FIG. 3 shows an arrangement of nozzles formed on a bottom face of anactuator chip 94;

FIG. 4 shows the principle of determining a correction value foradjusting misalignment, based on a test pattern;

FIG. 5 shows the printing results of the test pattern wherein only apart E1 of the ruled line shown in FIG. 4 is printed in such a mannerthat the printing position in a backward pass of main scan is shiftedfrom that in a forward pass in a sub-scanning direction;

FIG. 6 shows the printing results of the test pattern with all thenozzles in the nozzle array when the inclination of the nozzle array onthe backward pass is different from that on the forward pass;

FIG. 7 shows the printing results of the test pattern with only thenozzles in an upper end part of the nozzle array when the inclination ofthe nozzle array on the backward pass is different from that on theforward pass;

FIG. 8 shows the printing results of the test pattern with only thenozzles in a center part of the nozzle array when the inclination of thenozzle array on the backward pass is different from that on the forwardpass;

FIG. 9 is a flowchart showing a procedure of determining the correctionvalue based on the test pattern;

FIG. 10 is a block diagram illustrating the main configuration relatingto the correction of misalignment in the course of bidirectionalprinting in the first embodiment;

FIGS. 11(a) and 11(b) show a print heads having head assemblies where aplurality of nozzle units are arranged in the sub-scanning direction;and

FIG. 12 is a block diagram illustrating the main configuration relatingto the correction of misalignment in the course of bidirectionalprinting in a second embodiment.

BEST MODES OF CARRYING OUT THE INVENTION A. Structure of Apparatus

Some modes of carrying out the present invention are described below aspreferred embodiments. FIG. 1 schematically illustrates the structure ofa printing system including an ink jet printer 20 in a first embodimentof the present invention. The printer 20 includes a sub-scan mechanismthat drives a sheet feed motor 22 to feed a sheet of printing paper P ina sub-scanning direction, a main scan mechanism that drives a carriagemotor 24 to move a carriage 30 back and forth along an axis of a platen26 (in; a main scanning direction), a head driving mechanism that drivesa print head unit 60 (also referred to as “print head assembly”) mountedon the carriage 30 to control ejection of ink and creation of dots, anda control circuit 40 that controls transmission of signals to and fromthe sheet feed motor 22, the carriage motor 24, the print head unit 60,and a control panel 32. The control circuit 40 is connected to acomputer 88 via a connector 56.

The sub-scan mechanism for feeding the printing paper P has a gear train(not shown) that transmits the rotations of the sheet feed motor 22 tothe platen 26 and a sheet feed roller (not shown). The main scanmechanism for reciprocating the carriage 30 includes a sliding shaft 34that is arranged in parallel with the axis of the platen 26 to supportthe carriage 30 in a slidable manner, a pulley 38 that is combined withthe carriage motor 24 to support an endless drive belt 36 spannedtherebetween, and a position sensor 39 that detects the startingposition of the carriage 30. Another arrangement may alternatively beapplied for the main scan and the sub-scan. The printer carries out thebidirectional, reciprocating main scan to change the relative positionof the print head to the printing medium while ejecting ink from thenozzles and also performs the sub-scan between adjoining passes of themain scan to change the relative position of the print head to theprinting medium in a direction different from the direction of the mainscan, thereby effecting print on the printing medium.

FIG. 2 is a block diagram showing the structure of the control circuit40 included in the printer 20. The control circuit 40 is constructed asan arithmetic and logic operation circuit including a CPU 41, aprogrammable ROM (PROM) 43, a RAM 44, and a character generator (CG) 45that stores dot matrixes of characters. The control circuit 40 furtherincludes a dedicated I/F circuit 50 that dedicatedly works as aninterface with external elements like a motor, a head driver 52 thatconnects with the dedicated I/F circuit 50 and drives the print headunit 60 to eject ink, and a motor driver 54 that drives the sheet feedmotor 22 and the carriage motor 24. The dedicated I/F circuit 50includes a parallel interface circuit and receives a print signal PSsupplied from the computer 88 via the connector 56.

The following describes the structure of the print head unit 60. Theprint head unit 60 includes a housing in which an ink cartridge 70 forcontaining ink therein is housed, and a print head 28 that is amechanism of ejecting ink droplets. The whole configuration includingthe print head 28 and the housing for the ink cartridge 70 is called the“print head unit 60”, since the print head unit 60 is attached to anddetached from the printer 20 as an unitary part. Namely replacement ofthe print head 28 requires placement of the print head unit 60.

FIG. 3 shows an arrangement of nozzles formed on a bottom face of anactuator chip 94, which is disposed on a lower portion of the print head28. A color nozzle array and a black nozzle array are formed on thebottom face of the actuator chip 94 and are respectively aligned inlines in the sub-scanning direction. The term “actuator” here means anink ejection mechanism including nozzles and driving elements for inkejection (for example, piezoelectric elements or heaters). A nozzlesection in one actuator is generally molded as an integral ceramic part.Formation of two columns of nozzles in one actuator enables the highlyaccurate layout of nozzles and thereby improves the image quality. Inthe specification hereof, the “column of nozzles” is also be referred toas the “nozzle array”.

The black nozzle array includes 48 nozzles #K1 through #K48. Thesenozzles #K1 through #K48 are arranged at a fixed nozzle pitch k in thesub-scanning direction. The nozzle pitch k is equal to 6 dots in thisembodiment. The nozzle pitch k may be set to any other value that isobtained by multiplying the dot pitch on the printing medium P by anarbitrary integer of 2 or more.

The color nozzle array includes a yellow nozzle group 94Y, a magentanozzle group 94M, and a cyan nozzle group 94C. In the specificationhereof, the nozzle group of a colored ink is also referred to as the“chromatic nozzle group”. The yellow nozzle group 94Y includes 15nozzles #Y1 through #Y15. The pitch of these 15 nozzles is identicalwith the nozzle pitch k of the black nozzle array. This nozzle pitch kis also applied for the magenta nozzle group 94M and the cyan nozzlegroup 94C. A symbol “x” drawn between the lower end nozzle #Y15 of theyellow nozzle group 94Y and the upper end nozzle #M1 of the magentanozzle group 94M represents no presence of a nozzle at the position. Theinterval between the lower end nozzle #Y15 of the yellow nozzle group94Y and the upper end nozzle #M1 of the magenta nozzle group 94M isaccordingly double the nozzle pitch k. This double nozzle pitch is alsoapplied for the interval between the lower end nozzle #M15 of themagenta nozzle group 94M and the upper end nozzle #C1 of the cyan nozzlegroup 94C. In other words, the intervals between the respective nozzlegroups of yellow, magenta, and cyan are set to be double the nozzlepitch k.

Each of the nozzles in the color nozzle groups 94Y, 94M, and 94C islocated at an identical position in the sub-scanning direction with thecorresponding nozzle of the black nozzle array 94K. There are, however,no colored ink nozzles at the positions corresponding to the 16^(th),32^(nd), and 48^(th) nozzles #K16, #K32, and #K48 among the 48 nozzles#K1 through #K48 of the black nozzle array 94K.

In the course of printing, ink droplets are ejected from the respectivenozzles while the print head 28 is moving in the main scanning directiontogether with the carriage 30 (see FIG. 1). In some printing procedures,all the nozzles are not always used, but only part of the nozzles may beused.

B. Principle of Correcting Misalignment of Recording Positions

The bidirectional printing creates dots in both the forward pass and thebackward pass of the main scan to print one image on the printing mediumP. Accordingly, when ink is ejected aiming at the same recordingposition on the forward pass and on the backward pass of printing, dotsshould actually be recorded at the same position on the printing mediumP. This is because an image can be properly reproduced by combination ofdots formed in the forward pass and dots formed in the backward passonly when ink aiming at the same recording position actually forms dotsat the same position on the printing medium P.

As described previously, the recording positions in the main scanningdirection may be misaligned on the forward pass and the backward pass ofthe main scan, due to a backlash of the driving mechanism in the mainscanning direction or a warp of the platen that supports the printingmedium thereon. This method of correcting the misalignment of recordingpositions intentionally shifts the ejection timings of ink droplets onthe forward pass and on the backward pass from the “theoretical timingsto record dots at the same recording position.” This method accordinglyabsorbs the misalignment of recording positions and implements thecorrection, in order to ensure actual recording of dots at the samerecording position.

FIG. 4 shows the principle of determining a correction value foradjusting the misalignment based on a test pattern. The test pattern isprinted by causing the black nozzles #K1 through #K48 to create dots onthe printing medium P while the print head 28 is moving back and forthin the main scanning direction without any feed in the sub-scanningdirection. On the forward pass, ink droplets are ejected to form ruledlines on the printing medium P at fixed intervals in the sub-scanningdirection. In the example of FIG. 4, the solid lines with numerals 1through 8 allocated thereto represent the ruled lines printed on theforward pass.

On the backward pass, the ruled line is printed at different timings,that is, at several different printing positions, so that user canselect the “timing of recording a ruled line completely overlapping theruled line recorded on the forward pass”. In the example of FIG. 4, as amatter of convenience, the ruled lines printed on the backward pass areshown by the broken lines. In this example, the ruled line that isformed by ink droplets ejected on the backward pass at the “theoreticaltiming to record an identical ruled line” is the fourth left ruled line.The three left ruled lines from the third to the leftmost ruled line areprinted at delayed ejection timings of ink droplets, so that those areformed on the backward pass slightly shifted leftward from the ruledlines formed on the forward pass. The four right ruled lines from thefifth to the rightmost ruled line are printed at advanced ejectiontimings of ink droplets, on the other hand, so that the ruled lines areformed on the backward pass slightly shifted rightward from those formedon the forward pass. This results in the test pattern printed on theprinting medium P as shown in FIG. 4. The ruled lines Nos. 1 through 8printed on the backward pass are shifted rightward sequentially from theleft end by one dot pitch relative to the corresponding ruled linesprinted on the forward pass. The correction value is accordingly set asan integral multiple of the dot pitch. This example shifts the ruledlines printed on the backward pass by one dot pitch. In the case wherethe printing position of the ruled line is shifted by a finer unit, thecorrection value can be set as an integral multiple of the finer unit.In the example of FIG. 4, the ruled lines formed on the backward passare shown by the broken lines. This is only for the purpose ofdistinguishing the ruled lines formed on the backward pass from thoseformed on the forward pass and does not necessarily mean that the ruledlines are actually printed in broken line on the backward pass.

This procedure prints the ruled lines on the backward pass while varyingthe ejection timing of ink droplets to a plurality of different patternsin both the advancing and delaying directions from the theoreticalvalue. On the theoretical basis, the fourth left ruled line formed onthe backward pass is expected to be coincident with the ruled lineformed on the forward pass. In the actual state, however, as shown inFIG. 4, the fifth left ruled line formed on the backward pass (at theslightly advanced ejection timing of ink droplets relative to thetheoretical timing) is coincident with the ruled line formed on theforward pass. The ejection timing of ink droplets applied to form thefifth left ruled line enables dots to be actually recorded at the sameposition when ink is ejected aiming at the same recording position inboth the forward pass and the backward pass. This timing is stored asthe correction value and is applied for actual printing. Thisarrangement ensures the adequate correction of the recording positions.

This procedure of correction does not always require all the nozzles inthe nozzle array to be used for printing as discussed in FIG. 4. Onlypart of the nozzles in the nozzle array may be used to print the ruledlines, as long as it can determine whether or not the ruled lines drawnon the forward pass and on the backward pass form a single straightline. For example, only one of an upper end part E1, a center part C,and a lower end part E2 of the ruled line shown in FIG. 4 may be printedwith the corresponding part of nozzles as the test pattern. Thismodified procedure advantageously saves the quantity of ink required forprinting the test pattern.

The procedure of FIG. 4 prints the ruled lines overlapped on the forwardpass and on the backward pass. In the modified procedure, however, it ispreferable to move the print head 28 in the sub-scanning direction andthereby shift the printing position of the ruled line on the backwardpass from that on the forward pass. FIG. 5 shows the printing results ofthe test pattern wherein only the part E1 of the ruled line shown inFIG. 4 is printed in such a manner that the printing position on thebackward pass is shifted from that on the forward pass in thesub-scanning direction. As clearly understood from FIG. 5, the method,which shifts the printing position of the ruled line on the backwardpass from that on the forward pass in the sub-scanning direction,facilitates the determination of the degree of coincidence of the tworuled lines. In the example of FIG. 5, all the ruled lines formed inboth the forward pass and on the backward pass are shown by the solidlines.

The test pattern is not restricted to the vertical lines but may be anylinear pattern where dots are recorded intermittently.

C. Principle of Correcting Misalignment of Recording Positions inStructure where Inclination of Print Head in Backward Pass is Differentfrom that in Forward Pass

In the case where the print head 28 moves back and forth while keepingthe orientation of the nozzle array perpendicular to the main scanningdirection, the test pattern of FIG. 5 using only the nozzles in theupper end part of the nozzle array ensures the accurate determination ofthe correction value. In some cases, for example, due to the backlash ofthe driving mechanism in the main scanning direction, however, thenozzle array can not keep the orientation perpendicular to the mainscanning direction but may have different inclinations on the forwardpass and on the backward pass. In such cases, the procedure of printingthe test pattern corresponding to FIG. 4 on the forward pass and on thebackward pass gives the results of FIG. 6. FIG. 6 shows the printingresults of the test pattern with all the nozzles in the nozzle arraywhen the inclination of the nozzle array on the backward pass isdifferent from that on the forward pass.

In such cases, the procedure of printing the test pattern correspondingto FIG. 5 with part of the nozzles which create the dots of the upperend part E1 in the ruled line shown in FIG. 6 would give the results ofFIG. 7. In the results of FIG. 7, either the sixth left ruled lines orthe seventh left ruled lines are closest to a single straight line. Whenthe correction value for printing is determined based on the testpattern of FIG. 7, the ejection timing of ink droplets is thus regulatedto either the timing of the sixth left ruled lines or the seventh leftruled lines.

As clearly understood from the test pattern of FIG. 6 printed with allthe nozzles in the nozzle array, however, the printing result at eitherthe timing of the sixth left ruled lines or the timing of the seventhleft ruled lines has a relatively large misalignment of recordingpositions with regard to the lower end part E2 in the ruled line, whilehaving a small misalignment of recording positions with regard to theupper end part E1. Namely this procedure does not attain the optimumcorrection for the whole nozzle array. When the test pattern is printedwith part of the nozzles which create the dots of the lower end part E2in the ruled line shown in FIG. 6, on the other hand, either the thirdleft ruled lines or the fourth left ruled lines would be selected as theoptimum timing. In this case, a relatively large misalignment ofrecording positions is observed with regard to the upper end part E1.

The procedure of printing the test pattern corresponding to FIG. 5 usingpart of the nozzles which create dots of the center part C in the ruledline shown in FIG. 6 gives the results of FIG. 8. In the results of FIG.8, the fifth left ruled lines are closest to a single straight line.When the correction value for printing is determined based on the testpattern of FIG. 8, the ejection timing of ink droplets is thus regulatedto the timing of the fifth left ruled lines. As clearly understood fromthe test pattern of FIG. 6 printed with all the nozzles in the nozzlearray, in the case of the fifth left ruled lines, the optimum correctionis attained for the whole nozzle array. In this case, neither the upperend part E1 nor the lower end part E2 is widely deviated as the otherpart, but both the upper end part El and the lower end part E2 equallyhave relatively small deviations. In the procedure of the embodiment,the correction value, which is used to correct the misalignment ofrecording positions in the main scanning direction on the forward passand the backward pass, is determined with regard to a representativenozzle sub-array consisting of nozzles that are within a predeterminedrange in the vicinity of the center of the nozzle array.

D. First Embodiment

FIG. 9 is a flowchart showing a procedure of adjusting the positionalmisalignment. This adjustment is performed by the user in principle. Atstep S21, a test pattern for determining the correction value(positional misalignment test pattern) as shown in FIG. 8 is printedwith the printer 20. The method of printing the test pattern isdescribed in the section “C. Principle of Correcting Misalignment ofRecording Positions in Structure where Inclination of Print Head inBackward Pass is Different from that in Forward Pass”.

In the process of printing the test pattern, numerals representingmisalignment adjustment numbers (Nos. 1 to 8 in FIGS. 4 through 8) areactually printed above and below plural sets of vertical ruled lines.The misalignment adjustment numbers have the function as the correctioninformation representing the favorable correction state. Here theexpression of “favorable correction state” means the state giving aminimum positional misalignment of dots in the main scanning directionformed by the forward pass and by the backward pass, when the recordingposition (or the recording timing) of either the forward pass or thebackward pass is corrected with a proper correction value. In thedescription above, the misalignment adjustment numbers increase in anascending order from the leftmost end. Any numbers may be, however,allocated as long as the correction state can be specified.

In the step S22, the user observes the test pattern shown in FIG. 8 andinputs the misalignment adjustment number allocated to the set ofvertical ruled lines having the minimum positional misalignment on auser interface window (not shown) of the printer driver in the computer88 (see FIG. 2). The input misalignment adjustment number is stored intothe P-ROM 43 of the printer 20.

This series of procedure is carried out not only for the black nozzlearray 94K but for the color nozzle array 94YMC. Namely the P-ROM 43 ofthe printer 20 stores via the computer 88 (FIG. 2) a first adjustmentnumber representing a first correction value with regard to the colornozzle array 94YMC and a second adjustment number representing a secondcorrection value with regard to the black nozzle array 94K. The colornozzles are arrayed in the sub-scanning direction in the sequence of thecyan nozzle group 94C, the magenta nozzle group 94M, and the yellownozzle group 94Y as mentioned previously. The central magenta nozzlegroup 94M is used for printing the test pattern.

At step S23, when the user gives an instruction to execute printing.Then, at step S24, bidirectional printing is actually carried out whilethe positional misalignment is corrected with the correction value. FIG.10 is a block diagram illustrating the main configuration relating tothe correction of misalignment in the course of bidirectional printingin the first embodiment. The P-ROM 43 in the printer 20 includesadjustment number storage areas 202 a and 202 b and correction valuetables 206 a and 206 b. The first misalignment adjustment number isstored in the adjustment number storage area 202 a, and the secondmisalignment adjustment number is stored in the adjustment numberstorage area 202 b. Each of the correction value tables 206 a and 206 bstores the mapping of the misalignment adjustment numbers to the amountsof misalignment (that is, the correction values) of the recordingposition of the vertical ruled line formed by the backward pass in thetest pattern shown in FIG. 8.

The RAM 44 of the printer 20 stores a computer program having thefunction of a positional misalignment correction unit 210 to correct thepositional misalignment in the course of bidirectional printing. Thepositional misalignment correction unit 210 reads the correction valuescorresponding to the selected misalignment adjustment numbers from thecorrection value tables 206 a and 206 b in the P-ROM 43. On the backwardpass, the positional misalignment correction unit 210 receives a signalrepresenting the starting position of the carriage 30 from the positionsensor 39 (FIG. 1) and calculates the average of the first correctionvalue and the second correction value as a mean correction value. In thecase of color printing, the positional misalignment correction unit 210supplies a signal for specifying the recording timing of the print headto the head driver 52 according to the mean correction value. The headdriver 52 sends the driving signal to the actuator chip 94 and adjuststhe recording position on the backward pass to the specified recordingtiming transmitted from the positional misalignment correction unit 210.This causes the recording positions of both the black nozzle array andthe color nozzle array to be adjusted with the common mean correctionvalue. In the printing apparatus of the embodiment, the black nozzles aswell as the color nozzles are used for color printing.

In the case of monochromatic printing that does not use any color inks,it is preferable to correct the positional misalignment with only thesecond correction value. In a preferable application, when the computer88 (FIG. 1) informs the control circuit 40 of monochromatic printing,the control circuit 40 of the printer 20 (more specifically, thepositional misalignment correction unit 210 shown in FIG. 10) thuscorrects the positional misalignment in the course of bidirectionalprinting with only the second correction value.

As described above, the correction value, which is used to correct thepositional misalignment in the course of bidirectional printing, isestablished using the center nozzle part of the nozzle array as areference, and actually corrects the positional misalignment in thecourse of bidirectional printing with the correction value. Even whenthere is a difference in inclination of the nozzle array in the mainscanning direction between the forward pass and the backward pass, thisarrangement ensures the adequate correction for the whole nozzle array,thereby improving the print image quality.

In the case of color printing, the correction is carried out with themean of the respective correction values for the color nozzle array andthe black nozzle array (that is, the first correction value and thesecond correction value). In the case of monochromatic printing, on theother hand, the correction is carried out with only the correction valuefor the black nozzle array (that is, the second correction value). Thisarrangement ensures the optimum corrections in the respective printmodes.

In the structure of this embodiment, the color nozzles are arrayed inthe sub-scanning direction in the sequence of the cyan nozzle group 94C,the magenta nozzle group 94M, and the yellow nozzle group 94Y. Thecentral magenta nozzle group 94M is used for printing the test pattern.The correction is accordingly made to minimize the misalignment ofrecording positions of dots with regard to magenta. The misalignment ofrecording positions in magenta is more conspicuous than that in yellow.The yellow nozzle group 94Y, where the misalignment of recordingpositions in yellow is relatively inconspicuous, is allocated to one ofthe end parts that has the greater degree of misalignment of recordingpositions of dots than the center part. The misaligned recordingposition of the end part is accordingly not prominent. This arrangementmakes the total misalignment of recording positions of dots sufficientlyinconspicuous and thereby prevents deterioration of the image quality.In the structure of the embodiment, the magenta nozzles are used as therepresentative color nozzle sub-array to print the test pattern. Therepresentative color nozzle sub-array is, however, not restricted to themagenta nozzles. The cyan nozzles ensure the same effects as long as thecyan nozzles are located within the predetermined range in theneighborhood of the center of the nozzle array. The yellow nozzles mayalso be used as the representative color nozzle sub-array to correct themisalignment of recording positions.

In the structure of the embodiment, the representative nozzle sub-arrayused to print the test pattern includes nozzles located in the part Cshown in FIG. 6 among all the nozzles in the nozzle array. Therepresentative nozzle sub-array is, however, not restricted to this partC. For example, the representative nozzle sub-array may be a sequence ofnozzles that includes a specific nozzle closest to the center positionalong the nozzle array and consists of approximately half of all thenozzles in the nozzle array. More preferably the representative nozzlesub-array is a sequence of nozzles consisting of approximately one thirdof all the nozzles in the nozzle array. The representative nozzlesub-array is not required to have equal nozzles along the length of thenozzle array from the center position of the nozzle array. Therepresentative nozzle sub-array may be a group of nozzles that arelocated within a predetermined range in the neighborhood of the centerof the nozzle array. In other words, the representative nozzle sub-arrayincludes the specific nozzle closest to the center position along thenozzle array but does not include end nozzles on either end of thenozzle array.

In the case of color printing, the technique of the embodiment carriesout the correction with the simple arithmetic mean (average) of therespective correction values of the color nozzle array and the blacknozzle array (that is, the first correction value and the secondcorrection value). The calculation of the mean correction value is notrestricted to this method, but the mean correction value may be aweighted average of the first correction value and the second correctionvalue. This procedure may give weights to the first correction value andthe second correction value by taking into account the frequency of useof the color inks, yellow, cyan, and magenta, and the black ink, thedistance from the center of the nozzle array, and the degree ofconspicuousness of the misaligned recording position, and may calculatethe weighted average to obtain the mean correction value. The techniqueof the embodiment carries out the correction with the mean of the firstcorrection value and the second correction value in color printing. Whenblack nozzles are not used so often in color printing, the correctionmay be carried out with only the correction value for the color nozzlearray (that is, the first correction value) in color printing. Namelythe misalignment of recording positions in the main scanning directionmay be corrected in the course of bidirectional printing based on thefirst and the second correction values, without limitation on the way ofthe use of the first and the second correction values. The procedure ofusing the mean value of the first correction value and the secondcorrection value as discussed in the above embodiment is suitable forthe correction when black nozzles are used in color printing.

In this embodiment, the misalignment of recording positions is correctedin the printing apparatus having the print head unit 60 where oneactuator controls 48 nozzles arrayed in the sub-scanning direction asshown in FIG. 3. The technique of correcting the misalignment ofrecording positions according to the present invention is, however, notrestricted to the printing apparatus of this structure. As shown inFIGS. 11(a) and 11(b), the technique of the present invention isapplicable to another printing apparatus where a plurality of actuatorsare aligned in the sub-scanning direction and a large number of nozzlesNz are arranged in the sub-scanning direction. The print heads unitshown in FIGS. 11(a) and (b) have head assemblies 96 a through 96 d,where each head assembly includes a plurality of nozzle units arrayed inthe sub-scanning direction. The head assemblies 96 a through 96 drespectively eject the color inks of black (K), cyan (C), magenta (M),and yellow (Y). In the print head unit of this structure, therepresentative nozzle sub-array consists of nozzles Nzc that are locatedin a predetermined range in the vicinity of the center of the nozzlearray, which extends in the sub-scanning direction on the head assembly.The procedure prints the positional misalignment test pattern on theprinting medium with the representative nozzle sub-array, determines thecorrection value, and corrects the misalignment of recording positionswith the correction value. Since a long nozzle array extends in thesub-scanning direction, even a minute declination of the nozzle arrayresults in a significant positional misalignment on both ends of thenozzle array. The technique of correcting the misalignment of recordingpositions according to the present invention is especially effective forthe printing apparatus of such structure. The printing apparatus havingsuch a nozzle arrangement enables a large number of dots to besimultaneously created by one pass of the main scan and accordingly hasthe advantage of the high printing speed.

E. Second Embodiment

FIG. 12 is a block diagram illustrating the main configuration relatingto the correction of misalignment in the course of bidirectionalprinting in a second embodiment. The difference from the structure shownin FIG. 10 is that an actuator chip 95 a causing the black nozzle arrayto eject the black ink is separate from an actuator chip 95 b causingthe color nozzle array to eject the cyan, magenta, and yellow inks andthat a head driver 52 a for driving the actuator chip 95 a is separatefrom a head driver 52 b for driving the actuator chip 95 b. Namely thetwo head drivers 52 a and 52 b independently drive the respectiveactuator chips 95 a and 95 b. In this structure, the positionalmisalignment correction unit 210 gives the specified recording timingsindependently to the respective head drivers 52 a and 52 b. Thepositional misalignment in the course of bidirectional printing isaccordingly carried out for each actuator chip.

The feature of the second embodiment is that the correction value is setindependently for each actuator chip. The arrangement of correcting thepositional misalignment for each actuator chip ensures the finercorrection with regard to each nozzle group or groups in the sameactuator chip, thus further reducing the positional misalignment in thecourse of bidirectional printing.

The recording position of dots depends on a variation in ejection speedof ink droplets ejected from the nozzle array. Namely the variation inejection speed of ink droplets results in shifting the recordingposition of dots. The variation in ejection speed of ink dropletsejected from each nozzle array depends upon a variety of factors givenbelow:

(1) the manufacturing error of each actuator chip;

(2) the physical properties of ink (for example, the viscosity); and

(3) the weight of ink droplets.

If the main factor of the variation in ejection speed of ink droplets isthe manufacturing error of each actuator chip, the ink droplets ejectedfrom the same actuator have a substantially equal ejection speed. Inthis case, it is preferable to correct the misalignment of recordingpositions in the main scanning direction with regard to each of nozzlearray groups that are driven by different actuator chips respectively.

If the physical properties of ink and the weight of ink droplets alsosignificantly affect the ejection speed of ink droplets, it ispreferable to correct the misalignment of recording positions of dots inthe main scanning direction with regard to each ink or with regard toeach nozzle array.

The present invention is not restricted to the above embodiments ortheir modifications, but there may be many other modifications, changes,and alterations without departing from the scope or spirit of the maincharacteristics of the present invention. Some examples of possiblemodification are given below.

(1) It is preferable to set the correction value independently for eachnozzle group for which the ejection timing of ink droplets can becorrected independently. This arrangement further reduces the positionalmisalignment, compared with the arrangement of the embodiment discussedabove. The correction value may be set independently for each nozzlearray that ejects the same ink. For example, when two nozzle arrayseject the same specific ink, an identical correction value is appliedfor the two nozzle arrays.

(2) The technique of the embodiment adjusts the recording targetposition of dots (or the recording timing) in order to correct thepositional misalignment. Other methods may be applied to correct thepositional misalignment. One applicable procedure adjusts the frequencyof the driving signal transmitted to the actuator chip to correct thepositional misalignment.

(3) The technique of the embodiment adjusts the recording targetposition (or the recording timing) on the backward pass to correct thepositional misalignment. One modified procedure adjusts the recordingtarget position on the forward pass to correct the positionalmisalignment. Another modified procedure adjusts both the recordingtarget positions on the forward pass and on the backward pass to correctthe positional misalignment. In general, the positional misalignment iscorrected by adjusting at least one of the recording target positions onthe forward pass and on the backward pass.

(4) Although ink jet printers are described in the above embodiments,the present invention is not restricted to the ink jet printers but maybe applicable to a variety of printing apparatuses that generally carryout printing with a print head.

What is claimed is:
 1. A printing apparatus that carries outbidirectional, reciprocating main scan and prints on a printing medium,the printing apparatus comprising: a print head having a nozzle arraythat is arranged in a sub-scanning direction and ejects ink droplets torecord dots on the printing medium; a memory that stores a correctionvalue, the correction value being used to correct misalignment ofrecording positions in a main scanning direction between a forward passand a backward pass of the main scan; and a positional misalignmentcorrection unit that corrects using the correction value themisalignment of recording positions in the main scanning directionoccurring in bidirectional printing between the forward pass andbackward pass, wherein the correction value is determined according tocorrection information that represents a favorable correction stateselected based on a positional misalignment test pattern, the positionalmisalignment test pattern being printed on a printing medium with arepresentative nozzle sub-array, which is part of the nozzle array andis within a predetermined range around a center of the nozzle array, andwherein the representative nozzle sub-array is part of a full nozzlearray of the same color.
 2. A printing apparatus in accordance withclaim 1, wherein the nozzle array comprises: a color nozzle arrayincluding a plurality of color nozzles to eject color ink that arearranged in a predetermined sequence in the sub-scanning direction, anda black nozzle array including a plurality of black nozzles to ejectblack ink that are arranged in a predetermined sequence in thesub-scanning direction, and the memory stores a first correction valueand a second correction value, the first correction value being set forcorrecting the positional misalignment of recording positions in themain scanning direction on the forward pass and the backward pass of themain scan with regard to a representative color nozzle sub-array, whichis part of the color nozzle array and is within a predetermined rangearound a center of the color nozzle array, the second correction valuebeing set for correcting the positional misalignment of recordingpositions in the main scanning direction on the forward pass and thebackward pass of the main scan with regard to a representative blacknozzle sub-array, which is part of the black nozzle array and is withina predetermined range around a center of the black nozzle array.
 3. Aprinting apparatus in accordance with claim 2, wherein the positionalmisalignment correction unit corrects the misalignment of recordingpositions in the main scanning direction in the bidirectional printingwith a mean value of the first correction value and the secondcorrection value.
 4. A printing apparatus in accordance with claim 2,wherein the positional misalignment correction unit corrects themisalignment of recording positions with the first correction value in aprint mode in which the nozzles of the color nozzle array are used.
 5. Aprinting apparatus in accordance with claim 2, wherein the positionalmisalignment correction unit corrects the misalignment of recordingpositions with the second correction value in a print mode in which thenozzles of the color nozzle array are not used.
 6. A printing apparatusin accordance with claim 2, wherein the positional misalignmentcorrection unit corrects the misalignment of recording positions usingthe first correction value with regard to the color nozzle array, andcorrects the misalignment of recording positions using the secondcorrection value with regard to the black nozzle array.
 7. A printingapparatus in accordance with any one of claims 2 through 6, wherein thecolor nozzle array comprises yellow nozzles for ejecting yellow ink,cyan nozzles for ejecting cyan ink, and magenta nozzles for ejectingmagenta ink, and the representative color nozzle sub-array consists ofeither of cyan nozzles and magenta nozzles.
 8. In a printing apparatusthat comprises a print head having a nozzle array, which is arranged ina sub-scanning direction and ejects ink droplets to record dots on aprinting medium, and carries out bidirectional, reciprocating main scanto complete print on the printing medium, a method of correctingmisalignment of recording positions of ink droplets in a main scanningdirection between a forward pass and a backward pass of the main scan,the method comprising the steps of: (a) printing a positionalmisalignment test pattern on a printing medium with a representativenozzle sub-array, which is part of the nozzle array and is within apredetermined range around the center of the nozzle array; (b)determining a correction value according to correction information thatrepresents a favorable correction state selected based on the positionalmisalignment test pattern, the correction value being used to correctthe misalignment of recording positions in the main scanning directionon the forward pass and the backward pass of the main scan; and (c)correcting with the correction value the misalignment of recordingpositions in the main scanning direction in bidirectional printingbetween the forward pass and backward pass, wherein the representativenozzle sub-array is part of a full nozzle array of the same color.
 9. Arecording medium that stores a computer program for causing a computerto correct misalignment of recording positions of ink droplets in a mainscanning direction between a forward pass and a backward pass of themain scan, the computer connected to a printing apparatus that includesa print head having a nozzle array which is arranged in a sub-scanningdirection and ejects ink droplets to record dots on a printing medium,the printing apparatus carrying out bidirectional, reciprocating mainscan to print on the printing medium, the computer program causing thecomputer to attain the functions of: (a) printing a positionalmisalignment test pattern on a printing medium with a representativenozzle sub-array, which is part of a nozzle array and is within apredetermined range around the center of the nozzle array; (b)determining a correction value according to correction information thatrepresents a favorable correction state selected based on the positionalmisalignment test pattern, the correction value being used to correctthe misalignment of recording positions in the main scanning directionon the forward pass and the backward pass of the main scan; and (c)correcting with the correction value the misalignment of recordingpositions in the main scanning direction in bidirectional printingbetween the forward pass and backward pass, wherein the representativenozzle sub-array is part of a full nozzle array of the same color.
 10. Aprinting apparatus that carries out bidirectional, reciprocating mainscan and prints on a printing medium, the printing apparatus comprising:a print head having a nozzle array that is arranged in a sub-scanningdirection and ejects ink droplets to record dots on the printing medium;a memory that stores a correction value, the correction value being usedto correct misalignment of recording positions in a main scanningdirection on a forward pass and a backward pass of the main scan; and apositional misalignment correction unit that corrects using thecorrection value the misalignment of recording positions in the mainscanning direction occurring in bidirectional printing, wherein thecorrection value is determined according to correction information thatrepresents a favorable correction state selected based on a positionalmisalignment test pattern, the positional misalignment test patternbeing printed on a printing medium with a representative nozzlesub-array, which is part of the nozzle array and is within apredetermined range around a center of the nozzle array, wherein therepresentative nozzle sub-array is part of a full nozzle array of thesame color, and wherein the positional misalignment correction unitcorrects ejection timings of ink droplets from the representative nozzlesub-array and other nozzles in the nozzle array using the samecorrection value.
 11. In a printing apparatus that comprises a printhead having a nozzle array, which is arranged in a sub-scanningdirection and ejects ink droplets to record dots on a printing medium,and carries out bidirectional, reciprocating main scan to complete printon the printing medium, a method of correcting misalignment of recordingpositions of ink droplets in a main scanning direction on a forward passand a backward pass of the main scan, the method comprising the stepsof: (a) printing a positional misalignment test pattern on a printingmedium with a representative nozzle sub-array, which is part of thenozzle array and is within a predetermined range around the center ofthe nozzle array; (b) determining a correction value according tocorrection information that represents a favorable correction stateselected based on the positional misalignment test pattern, thecorrection value being used to correct the misalignment of recordingpositions in the main scanning direction on the forward pass and thebackward pass of the main scan; and (c) correcting with the correctionvalue the misalignment of recording positions in the main scanningdirection in bidirectional printing by correcting ejection timings ofink droplets from the representative nozzle sub-array and other nozzlesin the nozzle array, wherein the representative nozzle sub-array is partof a full nozzle array of the same color.
 12. A recording medium thatstores a computer program for causing a computer to correct misalignmentof recording positions of ink droplets in a main scanning direction on aforward pass and a backward pass of the main scan, the computerconnected to a printing apparatus that includes a print head having anozzle array which is arranged in a sub-scanning direction and ejectsink droplets to record dots on a printing medium, the printing apparatuscarrying out bidirectional, reciprocating main scan to print on theprinting medium, the computer program causing the computer to attain thefunctions of: (a) printing a positional misalignment test pattern on aprinting medium with a representative nozzle sub-array, which is part ofa nozzle array and is within a predetermined range around the center ofthe nozzle array; (b) determining a correction value according tocorrection information that represents a favorable correction stateselected based on the positional misalignment test pattern, thecorrection value being used to correct the misalignment of recordingpositions in the main scanning direction on the forward pass and thebackward pass of the main scan; and (c) correcting with the correctionvalue the misalignment of recording positions in the main scanningdirection in bidirectional printing by correcting ejection timings ofink droplets from the representative nozzle sub-array and other nozzlesin the nozzle array, wherein the representative nozzle sub-array is partof a full nozzle array of the same color.